The present disclosure is directed to an MWD acoustic measuring system. This disclosure utilizes a conventional drill collar construction affixed to a drill bit which enables the measurement of formation characteristics near the borehole. More specifically, it is an acoustic measurement device which uses the noise from the drilling device as the acoustic source. The acoustic signal in turn provides the information required to evaluate the formations near the borehole.
In drilling a well, a drill collar is a heavy section of tubing connecting the drill bit to the drill string. The drill collar provides the weight to force the drill bit into the formation, provides directional stability and is sometimes used because of its larger cross section as a location for sensors. As the well is drilled deeper, it becomes important to measure formation characteristics. It is especially important to measure characteristics of the formations next to the well borehole in a timely fashion, while drilling as opposed to using a wire line measurement after the hole has already been drilled to its maximum depth. One primary measurement of the formation is the sound velocity of compressional and shear waves. Another primary measurement is to determine the locations of boundaries between different types of formations. Acoustic measurements may be used to determine this information.
In a typical acoustic measurement tool, a sonic source is normally located at one point along the drill collar and transmits to another point along the drill collar. The acoustic energy radiates into the formation, travels along the borehole all the time some of the energy is going back into the borehole fluid. Receivers in the drill collar some distance from the transmitter, detect the energy in the fluid which comes from the formation to determine the sound velocity in the formation. Typically the shear wave velocity and compressional wave velocity in the formation is greater than the velocity of sound in the fluid. The differences in propagation times of the various signals allow their separation and measurement. It has also been demonstrated that the acoustic waves reflect from changes in the formation impedance.
During the drilling of a well, there are only a few different pathways for acoustic energy propagation. One pathway is through the formation. Another pathway is through the annular space in the well borehole on the exterior of the drill collar which is normally filled with drilling fluid. Also, there is another pathway which is through the drill collar. This steel column is a highly efficient medium for acoustic transmission. The sound velocity in this medium is also typically greater than the sound velocity in the formation. If there is an acoustic transmitter at some down hole location and an acoustic receiver at some distance from the transmitter, and both are relatively close to the well borehole, then it is not possible to transmit energy solely through the formation. Though techniques have been utilized to acoustically isolate the drill bit from the collars with some success, the coupling of the signal is not completely eliminated. The use of a mud motor will also effectively reduce but not eliminate the acoustic transmission to the drill collars.
This disclosure sets forth a system which can provide for the measurement of acoustic transmission in the formation and more specifically a system which enables the measurement of the acoustic transmission along a well borehole while the signal is also propagating in the drill collars. More specifically, this disclosure uses an acoustic source which is the drill bit itself. As the drill bit is rotated by the drill collar, the teeth on the drill bit bite into the face of the hole and make chiseling cuts. The energy of these cuts serves as an acoustic energy source. The acoustic energy will propagate through the formations parallel to the borehole and thus provide the desired pathway to each receiver. However, it is certain that a substantial portion of the acoustic energy that is formed by the drill bit operation will be transmitted up the drill collars to the receivers. Acoustic receivers are placed at two or more locations along the drill collars. The receive the acoustic energy propagating through the formation as well as the signal which propagates along the annulus between the drill collar and the formation. More importantly they receive the signal which propagates along the drill collar. This signal will typically be much larger than the signal which arrives from the formation or the annulus. The present disclosure sets forth a method and a procedure for data reduction so that the large drill collar conveyed signal will not interfere with the measurement of the formation signal.
This is accomplished in part by providing amplifiers at the acoustic receivers which clip the input signals to produce a sequence of 1's and 0's corresponding to the polarity of the received signal. The clipped signals are then cross correlated to produce a cross correlation function which describes the propagation velocities of various signal paths. The measured propagation velocities and the variability of the measured velocities allow separation of the desired values.